Fusobacteria (gram negative anarobic fusiform bacilli) have been implicated in periodontal diseases and have been isolated from brain and pulmonary abscesses. We have observed in our laboratory hemagglutination (HA) of washed untreated red blood cells of a variety of animal species (sheep, monkey, rabbit) and human blood types A positive, B positive, AB positive, O positive, A negative and AB negative by an antigen preparation of Fusobacterium polymorphum (f. nucleatum). F. fusiforms also recently included in F. nucleatum (Bergey's manual, 8th edition) did not cause hemagglutination of any of the animal red blood cells with similar antigen preparations. This observed characteristic of HA activity is being tested among oral and non-oral fusobacteria as a possible mechanism of taxonomic separation of these important microorganisms. Attempts at characterization of this agent by chemical, physical, and serologic means have revealed that HA activity to be in preparations of the sonicated cell walls of the organisms, the activity being heat labile (56 degrees C for 30 min.), precipitated by 33 percent (NH4)2SO4 saturation, and sensitive to acetone and phenol treatment. Human gamma globulin preparations and parotid saliva samples have been observed to inhibit the hemagglutination of sheep red blood cells by the F. polymorphum antigen preparations. The development of a hemagglutination inhibition test has shown the inhibition activity of the gamma preparation to be in the region of IgM elution by Sephadex G-200 chromatography. Attempts at isolation of specific antibody and serum and salivary glycoproteins which may be showing the observed inhibition are currently in progress. The cell receptor(s) for the observed HA activity appears specific for the antigen preparation and the binding of the organism to human epithelial cells and the inhibition of this binding by antibody or glycoproteins may be important in the colonization and the proposed pathogenic ability of these organisms in periodontal diseases.